OK, now that you know how to make a collaborative playlist, here comes the hard part: figuring out what that playlist should be about. You're going to need a theme.
A playlist theme can be anything, you make the rules (like one artist per song, seriously). Themes can be as simple as love, death, or how "tonight is the night and we only have tonight." A theme can follow a story arc like that of a relationship that starts out great and ends in disaster. Or it can just be "Metal Songs For Throwing Steel at The Gym."
But most importantly, a collaborative playlist theme should really feel more like a game with room to be creative (more of that later on).